The Good, the Bad and the Ugly: Optimizing OTC Injections

By Ute Albrecht, Caroline Tardivo, Larissa Nunes, Gabriel Pugina, Gerardo Moreno and Jasmine de Freitas

As growers are well into the second year of oxytetracycline (OTC) injections since approval for commercial use, more and more data are accumulating. The good news is that in all University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) trials, trees have been responding with significant improvements in yield and juice quality. The not so good news is that phytotoxic effects may occur. Researchers are still trying to figure out how to administer the injections without harming the trees in the process. 

THE GOOD

Let’s begin with the good news. Numerous field trials are in progress in commercial citrus groves. In all trials, yield increases of 25% to more than 100% have been measured consistently, along with significant improvements in fruit size and juice quality. While yield increases vary from trial to trial, the increase in Brix or total soluble solids appears to be more consistent, ranging from 10% to 20%.

In rootstock trials, all rootstocks responded positively to the injections but, perhaps not surprisingly, there were significant differences among them (Table 1). Although differences among rootstocks seem to be mostly a reflection of their general relative performance, OTC injections may help uncover their true potential, especially in trials where new rootstocks are included.

A comparison of rootstock versus scion injection did not produce any significant differences for yield and juice quality thus far. However, longer bark cracks are being observed when injecting into the scion. This year, OTC was taken up faster when injections were administered into the scion instead of the rootstock. This was not the case last year. In some cases, the uptake rate also varied by rootstock.

In some UF/IFAS trials, trees have been injected for two consecutive years. One of these trials is in a commercial grove near Fort Pierce. The trees in that trial are Valencia on sour orange rootstock. They were nine years old at the time of the first injection in 2022.

In the first year, a yield increase of 32% was measured when OTC was applied at the low rate (0.55 g/tree) and an increase of 67% when applied at the high rate (1.1 g/tree). The pounds solids per box were increased by 15% and 19%, respectively.

Greater increases were measured after two consecutive years of injection. Although yields were higher in 2024 compared to 2023 regardless of injection, compared to non-injected trees, OTC injected at the low rate increased the yield by 71% and at the high rate by 121%.

Pounds solids were increased by 15% regardless of the rate. The total pounds solids/acre was 1,066 for trees that received 0.55 g OTC and 1,385 for trees that received 1.1 g OTC, compared to 541 for the non-injected control trees (Table 2). The tree density in that trial was 290 trees per acre.

THE BAD AND THE UGLY

The bad (and sometimes ugly) news is that, especially in this year, phytotoxicity after injection seems to have been occurring with some regularity. This can manifest as moderate leaf yellowing in some parts of the canopy to severe yellowing and bronzing (Figure 1). In even more severe cases, leaf drop, twig dieback and fruit drop may occur.

However, such effects are usually only observed in a small section of the canopy. In most cases, yellowing or bronzing are followed by the production of healthy new leaves during the next flush.

The location of phytotoxicity in the canopy is related to the proximity of a branch to the injection site. Branches emerging closely in line with the injection site will usually receive more OTC than branches emerging on the opposite side of the injection and experience the most severe phytotoxicity.

At harvest, the injected side of the canopy usually looks healthier and produces more and better quality fruits than the opposite side (see citrusindustry.net/2024/05/14/address-sectoring-trunk-injected-trees). It must be noted that any xenobiotic (a chemical substance foreign to an organism) can potentially cause harm to the tree. Phytotoxic effects are not limited to OTC or the low pH of the injected solution. Similar or even worse phytotoxic effects with other injected chemicals, regardless of the pH, have been seen. Researchers have not observed any such effects when injecting water alone, even when it was acidified.

How can phytotoxic effects be avoided? Unfortunately, there is not a clear answer yet. This year, Florida had a long dry period followed by higher than usual temperatures. It is possible that the stress caused to the trees by these conditions rendered them more vulnerable to the additional stress from the injections.

Interestingly, a study initiated this year saw severe leaf yellowing in one grove location but not in another even though the trees were of similar age, and injections were performed at the same time using the same rate. This suggests that the local environment at the time of injection plays a role in the tree response. In one trial, leaf yellowing was observed even when the OTC was administered in two half doses on two opposite sides of the trunk. However, the leaf yellowing was more moderate than when administering a single injection of the full dose into one side of the trunk.

MOVING FORWARD

For new tree plantings, managing the trees so they have a longer trunk rather than a short one should be considered. The longer the trunk, the more time (distance) the OTC has to disperse evenly before reaching the scaffold branches. In any case, it is better to inject in line with the crotch of the scaffold branches instead of directly in line with a main branch.

The best approach is to use two injectors (or more for very large trees) spaced evenly around the trunk, to split the OTC into multiple smaller doses, instead of one injector on one side of the trunk containing a high dose (Figure 2). However, this may not be cost-effective and/or practical.

If deciding to split the injections, using a smaller injector tip may be considered. Using a smaller injection tip is especially important for injecting smaller trees, which are more prone to bark cracking and other trunk damage. A recently completed UF/IFAS study suggested that the number of injection sites is more important than the OTC dose. Chemjet injectors were used in that study (which require a 11/64-inch drill bit), but smaller tips are also available for the FLexInject injectors.

Researchers are constantly learning as studies continue. OTC injections are effective. Learning how to optimize the process without increasing the costs is the goal moving forward.

Acknowledgments: The authors thank the grower collaborators for providing trees, tree care and help with the harvest, and the Citrus Research and Development Foundation for financial support.

Ute Albrecht (ualbrecht@ufl.edu) is an associate professor; Caroline Tardivo, Gabriel Pugina and Larissa Nunes are PhD candidates; and Jasmine Freitas and Gerardo Moreno are biological scientists — all at the UF/IFAS Southwest Florida Research and Education Center in Immokalee.

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